Cat people! People who are cats, dressed like nuns! Rose would’ve never believed it if she hadn’t seen them with her own eyes. The Doctor tells her not to stare, but she can’t help it, they’re so incredibly brilliant!
They’re also up to something they shouldn’t be.
They mean well, they’re dedicated healers, doing everything they can to help their patients, but to do so, they’re using people, grown as lab rats to provide cures for those under the Sisterhood’s care.
It’s still wrong, so the Doctor frees and cures them. Now there’s a new human race, grown by cats.